1. Field of the Invention
The present invention relates generally to telecommunications systems and methods for reducing interference in cellular networks, and specifically to configuring the cell structure in cellular networks in order to reduce interference.
2. Background of the Present Invention
In modern cellular systems, the quality of the radio network highly depends upon the interference level in the network. The interference level is usually defined by the carrier to interference (C/I) ratio, which is the ratio of the level of the received desired signal to the level of the received undesired signal. The undesired signal can be a signal of the same frequency from a different cell (co-channel interference) or a signal of an adjacent frequency from a different cell (adjacent channel interference). In either case, the distribution of the C/I ratio throughout the network determines the type of frequency re-use pattern used in the network.
Since the number of frequencies available for cellular telecommunications are limited, frequency re-use patterns are necessary to provide cellular coverage to a geographic region. Frequency re-use is defined as the use of radio channels on the same carrier frequency, covering geographically different areas. Conventionally, these areas must be separated from each other by a sufficient distance in order to avoid co-channel interference.
However, various mechanisms, such as frequency hopping, power control and DTX, have been developed to reduce the interference in the cellular network without requiring an increase in the number of utilized frequency groups. Therefore, some cellular networks have been able to apply aggressive frequency re-use patterns, such as the 1/3 pattern, which uses only three frequency groups in a single site re-use pattern. These aggressive frequency re-use patterns also provide increased traffic capacity in the networks.
As is understood in the art, the traffic capacity in a cellular network can be increased by utilizing more frequencies or reducing the frequency re-use distance. If the number of available frequencies is limited, the only way to increase the capacity without building new sites is to reduce the frequency re-use distance. However, reducing the frequency re-use distance typically increases the interference in the network. Therefore, in addition to, or instead of, implementing an aggressive frequency re-use pattern, an overlaid/underlaid sub-cell structure can be introduced in order to increase the radio network capacity.
The overlaid/underlaid (OL/UL) sub-cell structure adds a second frequency re-use pattern to the cellular network with a shorter re-use distance than the existing re-use pattern. The cells using this second re-use pattern are typically restricted in size (lower power) to make a shorter re-use distance possible without creating excessive interference. These cells are termed overlaid sub-cells. The original cells that have overlaid cells associated with them are termed underlaid sub-cells.
This OL/UL sub-cell structure is created by dividing the available frequencies in the cellular network between the overlaid and underlaid sub-cells. Each overlaid sub-cell serves a smaller area than the corresponding underlaid sub-cell. Consequently, the number of frequencies per cell can be increased, thus providing an increased traffic capacity in the network without building new sites or adding more frequencies.
However, the OL/UL sub-cell structure only works when some of the mobile subscribers are positioned close to the base station. Therefore, in cases where many or all of the mobile subscribers are located near the corners of the cell, away from the base station, the OL/UL sub-cell structure may not provide any real benefit to the network operator or the mobile subscriber. Therefore, there is a need to adaptively switch between the OL/UL sub-cell structure and a normal cell structure, based on the relative position of the mobile subscribers in the cell.